Yes, this question was solved in year 2003/2004 by Moshe Dodo ben Barzilai. At that time, he authored a method of gematria called Mispar haBeka, where each letter of the Hebrew alphabet is "split" into its component letters and then these component letters are added up, giving a new sum to each letter. The letter Alef is comprised of two yuds and a vav, so its value is 26. The letter Bet is composed of three vavs, so its value is 18. Gimmel is composed of a zayin and a yud, etc.
Later, in year 2008, an author by the name of Jeffrey Meiliken posted a blog article using a similar method to try to find the 600,000 letters of Torah. Although, his source for the frequency of each letter in the Torah was wrong and some of the number components he used were questionable, his count came very close to 600,000. This blog was noticed by Moshe Dodo in 2010. Moshe checked his prior method of splitting the letters for gematria against the actual letters of Torah and found that it added up to exactly 600,000 letters. He had the solution for all that time and didn't realize it. He then began a discussion with Jeffrey Meiliken to compare their solutions.
These are some caveats to the solution. If we are to assume that the 600,000 letters correspond to the 600,000 Israelites (not including Levi who was not counted with them), we would expect to find a bit less than 600,000 letters. The reason for this is that all of the 600,000 Israelites died except for Yehoshua, Caleb, and the families of Dathan and Abiram who "went down alive into Sheol". Since Yehoshua and Caleb eventually died in the land of Israel, there remained alive several of the families of Dathan and Abiram, who were Reubenites. The solution proposed by Moshe Dodo ben Barzilai accounts for this and identifies the letters corresponding to each member of the families of Dathan and Abiram; he provides the location for these letters in accordance with the blessing of Moshe Rabbeinu to Reuben: "May Reuben live and not die. And may his population [Dathan, Abiram, and their male heirs] be included in the count." That is to say, even though every other letter corresponds to one of the Israelites that died, some of the letters corresponding to Reuben were originally written in black ink but they sunk into the Torah scroll. And they should be included in the full count of the letters.
The evidence for the sinking of certain letters into the skin of the Torah scroll is found in the words of Elisha, the prophet. In fact, the entire solution, the gematria which is the source of the solution, and the author of the gematria, is all found encoded in the words of Elisha in the section describing the bears and the 42 boys.
Jeffrey Meilikin's blog article and proposed solution: http://kabbalahsecrets.com/?p=110